An image sensor typically includes an array of pixels. When an image of a scene is to be captured by the image sensor, each pixel accumulates photo-generated charge based on the amount of light striking the pixel. The time in which the pixels collect photo-generated charge is known as the integration period. The pixels (or circuitry outside of the pixels) convert the image charge into image signals that are processed and combined to produce the image.
Pixels are non-ideal circuits that can experience varying amounts of dark current. Dark current represents charge that is accumulated by a pixel regardless of whether light is incident on the pixel or not. Dark current accumulates with, and is indistinguishable from, the image charge. The amount of dark current that accumulates in each pixel is dependent on several operating conditions, including the temperature of the image sensor, the length of the integration time, and the gain value(s) applied to the pixel signal when the pixel signal is readout of the pixel. Dark current typically increases as the temperature of the image sensor rises, usually doubling every +6 or +7 degrees Celsius. Additionally, the length of an integration period can affect how much dark current accumulates during an exposure period (the amount of time the pixels are exposed to light).
Each pixel can experience a slightly different amount of dark current. For a group of pixels, the distribution of the dark current typically follows a Poisson distribution (see e.g., 100 and 102 in FIG. 1). As the temperature of the image sensor increases, the length of the integration period increases, and/or the gain increases, the standard deviation of the dark current distribution increases. For example, as shown in FIG. 1, the standard deviation of the dark current distribution at a lower temperature, a short integration period, and/or a lower gain (see plot 100) is less than the standard deviation at a higher temperature, a longer integration period, and/or a higher gain (see plot 102).
The pixels in a digital image are typically represented with digital values or codes. To preserve the distribution of the dark current, a black level control circuit (or another component) typically controls the mean of the dark current such that the mean is maintained at some positive value within the digital code range. This positive level 104 is often referred to as the “pedestal” and is typically chosen so that the distribution at the worst operating condition (e.g., high temperature, high gain, and high integration period) prevents pixel signals from clipping at zero. The pedestal level 104 is then maintained at a fixed value under all operating conditions of the image sensor. The black level control circuit adds or subtracts an offset value to the dark current to make the mean of the dark current be substantially equal to the pedestal level 104.
As shown in FIG. 2, if pixel signals begin to clip at zero, the mean of the dark current 200 shifts because the negative pixels are not preserved. Further, the distribution of the dark current 202 no longer follows a Poisson distribution. The shifted mean 200 can result in color reproduction issues in images captured by the image sensor.
Additionally, when the accumulation of dark current is low due to a short integration period, a lower temperature, and/or a lower gain, the standard deviation is lower and the pedestal level 104 can be higher than necessary, which results in digital codes being wasted or not used (see e.g., plot 100 and area 106 in FIG. 1). When digital codes are wasted, the meaningful signal (the signal above the pedestal level 104) is supported by fewer digital codes than otherwise can be used, which may increase the quantization noise of the signal.